1. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 1 Measurement and simulation of the neutron response and detection efficiency of a Pb – scintillating fiber calorimeter A. Ferrari Fondazione CNAO (Milano)

2. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 2 The KLOE Pb-scintillating fiber calorimeter 1.2 mm 1.35 mm 1.0 mm Active material : 1.0 mm diameter scintillating fiber (Kuraray SCSF-81, Pol.Hi.Tech 0046), emitting in the blue-green region: Peak ~ 460 nm. Core: polystyrene,  =1.050 g/cm 3, n=1.6 High sampling structure: 200 layers of 0.5 mm grooved lead foils (95% Pb and 5% Bi). Glue: Bicron BC-600ML, 72% epoxy resin, 28% hardener. Lead:Fiber:Glue volume ratio = 42:48:10 Designed and put in operation as e.m. calorimeter Good performance in time and energy response:  (E)/E = 5.7 %/√E(GeV)  (t)= 54 ps/√E(GeV) and high photon efficiency see NIMA 482 (2002) 364-386

3. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 3 Why looking for neutron detection efficiency ?  Detection of neutrons of few to few hundreds of MeV is traditionally performed with organic scintillators (principle of operation: elastic neutron scattering on H atoms, with production of protons detected by the scintillator itself)  efficiency scales with thickness  ~1%/cm see C. Birattari, A.Ferrari, M.Pelliccioni et al., NIMA 297 (1990) 250-257, NIM A 338 (1994) 534-543 extended range rem counters  On the other hand, the extended range rem counters used in radiation protection are based on a structure scintillator/medium-high Z material, which enhances the neutron efficiency FLUKA  an intense Monte Carlo study has been performed with the FLUKA code, which is well validated for the hadronic physics, till the low energy region  an experimental test has been carried out with the neutron beam of the The Svedberg Laboratory of Uppsala (October 2006) [with TARI program support] FLUKA  an intense Monte Carlo study has been performed with the FLUKA code, which is well validated for the hadronic physics, till the low energy region  an experimental test has been carried out with the neutron beam of the The Svedberg Laboratory of Uppsala (October 2006) [with TARI program support] An estimate with KLOE data ( n are produced by K - interactions in the apparatus walls) gave:  for low energy neutrons (E kin ≤ 20 MeV), confirmed by KLOE MC ( expected:  10% ) - Measurement of the neutron e.m. form factors in the time-like region (DANTE) - Search for deeply bounded kaonic nuclei (AMADEUS) n are important for the DA  NE-2 program @ LNF

4. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 4 The neutron beam line at TSL  A quasi-monoenergetic neutron beam is produced in the reaction 7 Li(p,n) 7 Be.  42% of neutrons at the max energy  The absolute neutron flux in the peak is measured after the collimator by 2 monitors of the beam intensity. Accuracy: ~ 10% 5.31 m KLOE calorimeter module E KIN (MeV)

5. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 5 (1) (3) (2)  3 large data sets collected with different beam intensities: 1.5 kHz/cm 2, 3.0 kHz/cm 2 and 6.0 kHz/cm 2 The experimental setup and the data set ( 2 ) Beam position monitor: array of 7 scintillating counters, 1 cm thick. ( 1 ) Old prototype of the KLOE calorimeter: 60 cm long, 3 x 5 cells (4.2 x 4.2 cm 2 ), read out at both ends by Hamamatsu/Burle PMTs ( 3 ) Reference counter: NE110, 10×20 cm 2, 5 cm thick A rotating frame allows for: - vertical positions (data taking with n beam) - horizontal positions (calibration with cosmic rays) n Y X Z  For each configuration, several scans with different trigger thresholds  Typical run: 0.5-1.5 Mevents, 1.7 kHz DAQ rate  Cosmic rays run (beam off) for calibrations with MIPs. last plane not integrated in the acquisition system

6. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 6 The measurement of the global efficiency R NEUTRON : from beam monitor via neutron flux intensity measured by TSL. R TRIGGER : use coincidence between sides. Scintillator: T1 trig = Side 1×Side 2 Calorimeter: use the analog sum of 12 PMs/side (first four planes) T1 trig =  A×  B Global efficiency measurement integrated on the full spectrum I. The method f LIVE : live time fraction  : for preliminary measurement, assume full acceptance and no background  = R TRIGGER R NEUTRON × f LIVE × 

7. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 7 II. The scintillator efficiency  The measurement of the scintillator efficiency gives a cross calibration of the measurement method and of the beam monitor accuracy, with small corrections due to the live time fraction  The energy scale was calibrated with a 90 Sr  source. 10% accuracy for horizontal scale (threshold) and the vertical one (  ) Threshold (MeV e  equiv. energy)  (%) Results agree with “thumb rule” (1%/cm): 5% for 5 cm thick scintillator (with a threshold of  2.5 MeV electron equivalent energy) Agreement within errors with previous published measurements in the same energy range, after a rescaling of them to our thickness

8. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 8 III. The calorimeter efficiency  Energy scale setting done by MIP calibration of all channels, and using the MIP/MeV scale factor used in KLOE  10% uncertainty on both horizontal and vertical scales  Stability wrt very different run conditions: a factor 4 variations of both live time fraction (e.g. f LIVE =0.2  0.8) and beam intensity (1.5  6.0 kHz/cm 2 ). Very high efficiency Very high efficiency, about 4 times larger than the expected if only the amount of scintillator is taken into account: ~ 8% for 8 cm of scintillating fibers. Very high efficiency Very high efficiency, about 4 times larger than the expected if only the amount of scintillator is taken into account: ~ 8% for 8 cm of scintillating fibers.  (%) Compare with our scintillator efficiency measurement, scaled by the scintillator ratio factor 8/5 Thr (MeV e  equiv. energy)

9. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 9 The neutron spectrum from ToF n  Correct raw spectra for T0 and convert into ns  Since the trigger is phase locked with the RF ( time structure: 45 ns), rephasing is needed for neutrons with E kin < 50 MeV (5.3 m far from the target) ToF (ns)  From ToF spectrum obtain  of the neutron  Assuming neutron mass, obtain E kin

10. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 10 Energy vs ToF Energy released vs ToF ToF (ns) Energy (MeV eq. el.) threshold: 15 mV Charge response  The collected charge is here expressed as the energy of an electron that gives the same charge response

11. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 11 LEAD GLUE FIBERS base module replicas 200 layers  Using the FLUKA tool LATTICE the fiber structure of the whole calorimeter module has been designed.  In the base module the calorimeter is simulated in detail, both under the geometrical point of view and with respect to the used materials The calorimeter simulation with FLUKA  All the compounds have been carefully simulated. - for the fibers, an average density between cladding and core has been used : ρ = 1.044 g/cm 3 - glue: 72% epoxy resin C 2 H 4 O,  =1.14 g/cm 3, + 28% hardener,  =0.95 g/cm3 PolyoxypropylediamineC 7 H 20 NO 3 90% TriethanolamineC 6 H 15 NO 3 7% AminoethylpiperazineC 6 H 20 N 3 1.5% DiethylenediamineC 4 H 10 N 2 1.5% hardener composition

12. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 12 The readout simulation The simulation of the Birks effect Fluka gives energy deposits in the fiber. The light is propagated by hand at the end of the fiber taking into account the attenuation. The energy read-out has been simulated by including:  the generation of photoelectrons  the constant fraction distribution  the discriminator threshold.  No trigger simulation is included at the moment. dL/dx = k dE/dx / [ 1 + c 1 dE/dx + c 2 (dE/dx) 2 ] c 1 = 0.013 c 2 = 9.6×10 -6 The energy deposits are computed in Fluka taking into account the Birks effect, that is the saturation of the light output of a scintillating material when the energy release is high, due to the quenching interactions between the excited molecules along the path of incident particles: In literature and in GEANT:

13. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 13 Proton beam Li target n 5.5° The simulation of the beam line Z(cm) Y(cm) Shielding (concrete and steel) Calorimeter 7 Li Target Gaussian angular distribution (Journal of Nuclear Science and Technology, supplement 2(2002), 112-115 ) At the Li-target At the calorimeter E kin (MeV)  The beam line has been simulated starting from the neutrons out of the Litium target At the entrance of the beam monitor

14. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 14 Neutron interactions in the calorimeter Each primary neutron has a high probability to have elastic/inelastic scattering in Pb target P el (%) P inel (%) Pb 32.6 31.4 fibers 10.4 7.0 glue 2.3 2.2 In average, secondaries generated in inelastic interactions are 5.4 per primary neutron, counting only neutrons above 19.6 MeV. neutrons above 19.MeV 62.2% photons 26.9% protons6.8% He-43.2% deuteron 0.4% triton 0.2% He-3 0.2% Typical reactions on lead: n Pb  x n  y   Pb n Pb  x n  y   p + residual nucleus n Pb  x n  y   p + residual nucleus In addition, secondaries created in interactions of low energy neutrons (below 19.6 MeV) are - in average - 97.7 particles per primary neutron. neutrons 94.2% protons 4.7% photons 1.1% Simulated neutron beam: E kin = 180 MeV

15. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 15 A typical inelastic process n Z(cm) p n1n1 n2n2 n3n3 n4n4 X(cm) primary vertex E n = 175.7 MeVE n (p) = 126 MeV The enhancement of the efficiency appears to be due to the huge inelastic production of neutrons on the lead planes. These secondary neutrons: - are produced isotropically; - are produced with a non negligible fraction of e.m. energy and of protons, which can be detected in the nearby fibers; - have a lower energy and then a larger probability to do new interactions in the calorimeter with neutron/proton/γ production.

16. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 16 Neutron yield inside the calorimeter X(cm ) Z(cm) beam Neutron fluence E kin (MeV)  (E) cos( θ ) dN/dΩ (n sr -1 per prim) 1° plane 4° plane Isotropic angular distributions from inelastic scattering Energy distribution Energy distribution

17. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 17 The proton yield inside the calorimeter X(cm) Z(cm) beam Proton fluence  (E) E kin (MeV) cos( θ ) dN/dΩ (prot sr -1 per prim) Protons are mainly concentrated along the direction of the primary beam Energy distribution Energy distribution Angular distribution Angular distribution

18. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 18 A key point: the high sampling frequency The energy deposits of the ionizing particles (protons and excited nuclei) are distributed mainly in the nearby fibers: the high sampling frequency is crucial in optimizing the calorimeter Z deposit –Z prim.vert  (cm) X deposit -X prim.vert (cm) neutron lateral profile neutron lateral profile proton lateral profile proton lateral profile Interaction vertex Interaction vertex in lead in lead (protons and res nuclei) (protons and res nuclei) Interaction vertex Interaction vertex in lead in lead (protons and res nuclei) (protons and res nuclei)

19. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 19 E kin (GeV) Protons Neutrons E.m. energy Others E (ril) /E (tot) (ril) Particle contribution to the energy response E (tot) (ril) = Σ E p (ril) + Σ E n (ril) + ΣE em (ril) + ΣE resnuc (ril) E (tot) (ril) = Σ E p (ril) + Σ E n (ril) + Σ  E em (ril) + ΣE resnuc (ril) Particle contribution to the energy released in the fibers: Evaluating the particle contribution to the energy response, we have to take into account: - the contribution of the highly ionizing particles: protons and excited nuclei; - the contribution of the e.m. energy The neutron contribution is not to take into account in general, because the neutrons transfer energy to the nuclei of the fibers basically as invisible energy. We don’t know if at least a part of this energy can produce somehow scintillating photons. For this reasons, we evaluate first the efficiency without taking into account the neutron energy deposits, then we do the exercise to re-calculate the efficiency also including the neutron contribution, as a superior limit to the true value.

20. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 20 Integrated efficiency: 50% The simulated efficiency vs energy ε (%) E kin (MeV) PreliminaryPreliminary  No cut in released energy!  No trigger simulation  Simulation of the discriminator threshold applied only at the cell level not at the cluster level  By taking into account neutron energy deposits: ε ≈ 56% To be read as a superior limit !!

21. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 21 Data vs Monte Carlo  The whole cluster algorithm procedure is under study  A first comparison at the cell level has been made (in this example: threshold = 15 mV) The agreement Data/MC is good, except for the lower energy region n To be included in the simulation: local shielding, metallic supports, … to simulate the background due to the neutrons scattered on the materials in the experimental hall

22. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 22 T cell (ns) MC ● Exp E kin (MeV) ε(%) A study on the detection efficiency vs energy A study on the detection efficiency vs energy A fast Monte Carlo procedure has been used to test the sensitivity of the time distribution to the shape of the efficiency curve n two efficiency functions (Fermi-Dirac) are used in Monte carlo generation  time distributions for the central cell in the first plane are calculated and compared with the experimental data (threshold = 15 mV) In this example : T cell (ns) MC ● Exp E kin (MeV) ε(%)

23. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 23Conclusions We think that this work is the starting point for the study and the development of a new, compact, cheap, fast and efficient neutron detector  A first comparison Data/Monte Carlo has been done and is satisfactory. Work is in progress to tune the Monte Carlo.  The first measurement of the detection efficiency of a high sampling lead-scintillating fiber calorimeter to neutrons, in the kinetic energy range [20, 178] MeV has been performed at The Svedberg Laboratory, Uppsala. The efficiency integrated over the whole neutron energy spectrum ranges between 40% and 50%, at the lowest trigger threshold used.  A detailed Monte Carlo study, carried out with FLUKA, showed that that the origin of a such enhancement is related both to an effect shower-like, due to the inelastic processes in the Pb-scintillating fiber structure of the calorimeter, AND to the high sampling fraction used of this detector.  New tests on neutron beam in different energy regions are in program

24. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 24 Some additional information…

25. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 25 Details on DAQ Scintillator trigger: Side 1 – Side 2 coincidence (T1=S1×S2) Calorimeter trigger: based on analog sum of the signals of the first 4 plan out of 5 (T1=  A×  B). Trigger signal is phase locked with RF signal (T1 free). Vetoed from retriggering by a 5-35 ms busy signal and by the DAQ busy. The final trigger signal is: T2 = T1free.AND.NOT(BUSY). T1free, T2, and the n monitor signals are acquired by a scaler asynchronous from DAQ. Fraction of live time: T2/T1free; essential for the efficiency evaluation. T2/T1 FREE Thr (mV) Neutron rate proportional to neutron monitor via neutron flux intensity (I 0 ) and peak fraction (f P ) An absolute rate calibration should be provided by scintillator counter. Calorimeter scintillator efficiency rate is almost independent from beam monitor.

26. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 26 Time structure 4.2 ms 2.4 ms 40 ns 41 ns  5 ns FWHM RF Macro structure Calorimeter Trigger signal

27. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 27 Test of phase locking Beam RF T1(Free) Test done with a random trigger at 60KHz

28. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 28 S 1 (ADC counts) Thr (mV) 1.15 count/mV Trigger threshold calibration: mV to ADC counts Scintillator calibration  source to set the energy scale in MeV: 90 Sr   endpoint 0.564 MeV; 90 Y   endpoint 2.238 MeV. Fit of the b spectrum, with ADC counts to MeV factor as a free parameter. 0.021 MeV/count S1S1 ADC counts

29. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 29 Calorimeter calibration  A  1.6 count/mV  B  2.0 count/mV ADC counts mV Cell response equalized: MIP peak at  600 ADC counts. Trigger threshold calibration: - HP attenuators used for  A and  B not to exceed the dynamic range of the ADC; different attenuation factors: f A =2.0, f B =1.7. - Threshold in counts studied with different methods. Energy scale set with MIPs using the conversion factor from KLOE: a MIP in a calorimeter cell corresponds to an electron of 35 MeV.

30. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 30 100 mV 40 mV 15 mV TOF distributions with different trigger thresholds

31. Anna Ferrari VCI 2007, Wien, February 22nd, 2007 31 Simulation of the energy read-out fiber (active material) energy deposit given by FLUKA The light is propagated by hand at the end of the fiber using the parametrization: Kuraray Politech The number of photoelectrons generated by the light collected by each fiber is evaluated: Attenuation generated according to a Poisson distribution the constant fraction distribution is simulated (15% fr., 10 ns t.w.) to obtain the time E a,b (fib) = E (dep) ·[0.35 e -x(a,b)/50 + (1- 0.35) e –x(a,b)/430 ] E a,b (fib) = E (dep) ·[0.35 e -x(a,b)/50 + (1- 0.35) e –x(a,b)/330 ] t a,b (fib) = t (dep) + X (a,b) /17.09 n a,b (pe-fib) =E (fib) (MeV) (a,b) · 25 t (a,b) (p.e.) = t (a,b) (fib) + t scin + 1 ns (smearing) n a,b (pe-cell) = ∑ t(pe)<300 ns